This invention relates to memories with redundant lines of memory cells, and more particularly to MOS circuits for implementing the replacement of a defective line of memory cells with a redundant line of memory cells.
With the discovery that yield in the manufacture of memories can be enhanced by the use of redundant lines of memory cells, several techniques have been developed for replacing a defective line of memory cells with a redundant line of memory cells. One technique uses a laser to open circuits which will result in such a replacement. One advantage of the laser technique is that very little additional circuitry is required. Another advantage is that there are no additional propagation delays between the address inputs and the decoder which enables the line of memory cells which is selected by a particular address. Disadvantages of the laser technique include a large capital expense for the equipment and a separate alignment for each circuit that must be opened. Fusible links which provide an open or closed circuit in response to an electrical signal are also available. The fusible link can be, for example, a polysilicon fuse or a floating gate transistor. Polysilicon fuses are particularly useful in bipolar technology because of the availability of relatively high current. Enough current to open a polysilicon fuse is easily carried by a bipolar device without requiring more than the power supply voltage. For insulated gate field effect transistor (IGFET) technology, commonly known as MOS technology, polysilicon fuses require a special high voltage during the probe stage to generate sufficient current. In addition the current carrying transistors must be made to be very large in order to carry the required current even when driven at the high voltage. The floating gate technique does require a special high voltage during the probe stage, but does not require the large amount of current which is required to open a polysilicon fuse. Although techniques exist for erasing floating gate transistors which have been programmed, such erasing techniques are not particularly relevant to redundancy techniques. The floating gate as used is thus analogous to a fuse in that prior to being programmed it provides a current path but after being programmed it provides only an open circuit. The primary advantage of the fusible link technique is that minimal additional capital expense is required. One disadvantage is that additional circuitry for implementing the replacement is required. Another disadvantage, in MOS technology, is that there is some additional circuitry between the address inputs and the decoder for steering or comparing or for some other purpose relating to implementing replacement of a defective line of memory cells. Such additional circuitry adds one or more propagation delays which slow down access time of the memory. U.S. Pat. No. 4,250,570, Tsang et al, does disclose a bipolar circuit applicable to bipolar decoders which avoids this additional circuitry and consequent propagation delays. Another disadvantage of fusible link techniques is that replacement of a defective line of cells with a redundant line of cells requires separate steps for the various fusible links. Even U.S. Pat. No. 4,250,570 suggests that each polysilicon fuse to be opened should be opened in a separate step because of the large current requirements.